1. Field of the Invention
The present invention relates to an imaging-optical system in which zooming is performed by moving at least one lens group thereof without varying a distance from the most object-side surface of the first lens group to the imaging plane. With the above arrangement, the imaging-optical system enables both normal viewing with a lower magnification ratio at the short focal length extremity and magnified viewing at the maximum magnification position.
2. Description of the Prior Art
As an imaging-optical system which enables both normal viewing and magnified viewing, an endoscope viewing-optical system in which at least one lens group is moved to perform zooming has been known in the art, as disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) No. Hei-11-295596.
More specifically, in JUPP No. Hei-11-295596, an imaging device and at least one lens group are made movable; however, due to this arrangement, a signal processor board and cables have to be connected to the imaging device such as a CCD, so that a strong driving force is required in order to move the imaging device. In the case where the imaging device is driven by an actuator or a motor, it becomes especially significant to reduce the amount of load from the driven components (the imaging device and at least one lens group). Accordingly, it is desirable to maintain the CCD (imaging device) stationary regardless of a zooming operation.
In addition, JUPP No. 2001-166203 and JUPP No. 2001-91832 are taken as examples of prior art in which a plurality of lens groups are moved in order to perform zooming:
JUPP No. 2001-166203 has disclosed an optical system of a three-lens-group arrangement, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), a second lens group having a positive refractive power (hereinafter, a positive second lens group) and a third lens group having a positive refractive power (hereinafter, a positive third lens group) in this order from the object. The positive second lens group and the positive third lens group are moved to perform zooming. However, field curvature (astigmatism) and lateral chromatic aberration are extremely larger at magnified viewing; and
JUPP No. 2001-91832 has disclosed an optical system of a four-lens-group arrangement, i.e., a negative fist lens group, a positive second lens group, a negative third lens group and a fourth lens group having a positive refractive power (hereinafter, a positive fourth lens group) in this order from the object. The negative third lens group and the positive second lens group or the positive fourth lens group are moved to perform zooming. However, a refractive power (hereinafter, power) of the negative third lens group is strong, so that the number of lens elements for the correcting of aberrations becomes larger. Consequently, the overall length of the optical system becomes longer.
In addition, it has been known that a retrofocus-type optical system, which has a smaller number of lens elements, has been used in an endoscope. However, there is a large change in optical performance between normal viewing and magnified viewing. In particular, lateral chromatic aberration largely varies between normal viewing and magnified viewing; and similarly, astigmatism largely varies therebetween. Therefore it is difficult to balance lateral chromatic aberration and astigmatism at normal viewing and magnified viewing. For example, if lateral chromatic aberration is suitably corrected at normal viewing, the same at magnified viewing is overcorrected; while, if lateral chromatic aberration is suitably corrected at magnified viewing, the same at normal viewing is undercorrected. Namely, if the correcting of aberration is suitable made at one of normal viewing and magnified viewing, resolution of an image at the other of normal viewing and magnified viewing deteriorates, or vice versa.
In recent years, higher picture quality with a larger number of pixels has been desired due to the further development of CCDs.
Accordingly, the size of each single CCD-pixel has become smaller; and an objective optical system has been required to have higher optical performance at both normal viewing and magnified viewing.